The present invention relates to fuel cell systems for producing electricity from an electrochemical reaction, and more particularly to a low humidification membrane of such fuel cell systems.
Fuel cell systems often include fuel processors that produce hydrogen from hydrocarbon fuel. Fuel cell systems typically include a plurality of fuel cells that produce electricity from the conversion of electrochemical energy resulting from the reaction of reducing and oxidizing agents (e.g., hydrogen and an oxidant).
Fuel cells have been used as a power source in many applications and can provide improved efficiency, reliability, durability, cost and environmental benefits over other sources of electrical energy. As a result of the improved operation of these fuel cells over other sources of energy, and in particular, the reduced emissions (i.e., practically zero harmful emissions), electric motors powered by fuel cells for use in cars and other vehicles to replace internal combustion engines are very attractive.
One common type of fuel cell is a proton exchange membrane (PEM) fuel cell, which employs a thin polymer membrane that is permeable to protons, but not electrons. The membrane in the PEM fuel cell is part of a membrane electrode assembly (MEA) having an anode on one face of the membrane and a cathode on the opposite face. The membrane is typically made from an ion exchange resin such as a perfluoronated sulfonic acid. The MEA is sandwiched between a pair of electrically conductive elements that serve as current collectors for the anode and cathode, and contain appropriate channels and/or openings for distribution of the gaseous reactants of the fuel cell over the surfaces of the respective anode and cathode catalysts.
In PEM fuel cells, hydrogen (H2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (i.e., O2), or air (i.e., a mixture of O2 and N2), or O2 in combination with other gases. The anode and cathode typically comprise finely divided catalytic particles supported on carbon particles, and admixed with a proton conductive resin. The catalytic particles are typically precious metal particles, such as, for example, platinum.
These MEAs also require controlled operating conditions in order to improve operation efficiency and prevent degradation of the membrane and catalysts. These operating conditions include proper water management and humidification. In particular, if a proper moisture level of the electrolyte membrane is not maintained, cell performance is affected (i.e., proton conductivity is reduced and the current produced by the cell drops). Failure to control water levels of the membrane may prevent the membrane from properly conducting hydrogen ions, thereby resulting in a drop in power produced by the fuel cell. For example, if the cell is too dry, protonic conductivity is reduced. Conversely, if liquid water remains in the fuel cell at the cathode, oxygen is unable to penetrate the water remaining and reach the cathode catalyst, thereby also reducing fuel cell performance.
Prior fuel cell systems typically utilize an externally humidified air stream to maintain the proper moisture level of the membranes of the MEA. However, providing water to the stack is costly from a system point of view, and it is desirable to supply as little water as possible in order to minimize system complexity and cost.
Accordingly, the present invention provides adsorbent particles embedded in the membrane which adsorb water under wet conditions and provide a reservoir of water to keep the membrane irrigated under dry conditions. Thus, the water adsorbing particles allow the fuel cell to survive periods of xe2x80x9cinlet-stream draughtxe2x80x9d without excessive loss in conductivity. A hydrogen oxidation catalyst is supported on the water adsorbent material in order to catalyze the reaction of hydrogen and oxygen that are crossing through the membrane. This reaction forms water and will serve to irrigate the water adsorbent particles provided within the membrane. With the present invention, humidification requirements of a fuel cell stack in an operating system will be reduced. This will result in reduction or elimination of humidification equipment and the reduction or elimination of condensing requirements downstream of the stack. System complexity and cost are also substantially decreased. Fuel cell stack response to periods of low-inlet-stream humidity will be greatly improved while membrane durability will also be improved.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.